Rigify - what are tweak bones,and what do they do?
They allow you to deform your character with a lot of freedom.
This can be useful for cartoony animation... although if you don't go too extreme with them, you could possibly use them to create realistic motion.
I have a video where I explain a bit about them:
https://www.youtube.com/watch?v=vKgH5zXIYmM&amp;feature=youtu.be&amp;t=1171